A magnetic disk drive, such as a hard disk drive, stores data on one or more disks coated with a magnetic medium. For read/write purposes, the surface of the magnetic medium carries a number of generally parallel data tracks, which on a disk type medium, are arranged concentrically with one another about the center of the disk.
An actuator arm positions a transducer or “head” over a desired track, and the head writes data to the track or reads data from the track. As the disk rotates, the actuator arm moves the head in a radial direction across the data tracks under control of a closed-loop servo system, based on position information or “servo data”, which is stored within dedicated servo fields of the magnetic medium of the disk. The servo fields can be interleaved with data sectors on the disk surface or can be located on a separate disk surface that is dedicated to storing servo information. As the head passes over the servo fields, it generates a feedback signal that identifies the location of the head relative to the centerline of the desired track. Based on this location, the servo system moves the actuator arm to adjust the head's position so that it moves toward a position over the desired track and/or a desired location within the track of current interest.
One requirement in the manufacture of such a hard disk drive relates to the formation of the servo patterns on the magnetic disk, which are typically in concentric circular patterns. Mastering Systems for forming the servo tracks on a master stamper used in magnetic contact printing have used both stepped translation mechanisms with laser beams and continuous translation mechanisms with electron beams. During the recording of the servo data on the substrate, the substrate is located on a rotating turntable located at the top of a spindle. A spindle control motor rotates the spindle in accordance with control signals provided by a controller. The servo tracks are recorded through exposure to an electron beam or laser beam.
A format signal generator is used to control the electron beam generator or laser beam generator to form the pattern on the disk as it is rotating with the turntable. The control of the format signal generator, and hence the recording on the disk, may be made in accordance with encoder signals from an encoder located at the motor.
Typically, the encoder is provided at a bottom end of a spindle, the end opposite to that of the spindle on which the turntable is mounted. A precise motor control is provided by employing an encoder located in this position. In other words, the encoder at the bottom of the spindle may be mounted such that there is substantially no eccentricity with respect to the axis of rotation of the spindle. This allows for a very precise control of the motor based on the encoder signals.
Although providing for a precisely centered mounting and accurate motor control signals, the location of the encoder at the bottom of the motor is problematic when used to provide a clock, position or velocity source for the format signal generation process during recording. This is due to the mechanical vibrations, however slight, that occur in the rotating portions of the recording system. In particular, the vibrations at the top of the spindle, where the turntable is located, are not synchronized with the vibrations at the bottom of the spindle, where the encoder is located. Because the distances employed in servo tracks are extremely small, such as between 50 to 90 nm, even minute disturbances will create problems of track-to-track phase errors.
Simply moving the encoder adjacent to the recording surface at the top of the spindle does not provide an adequate solution, however. This is because, in practice, providing for a substantially perfectly centrically mounted encoder adjacent to the recording surface has proven to be very difficult to achieve. Hence, an encoder provided adjacent to the recording surface at the turntable exhibits eccentricity during rotation so that the signals are inadequate for providing a precise clock, position or velocity signal employed to control recording.